Basically Shetland sheep have three sets of colour genes.
Set 1 (called the Agouti set), In order of dominance, most dominant first:
White, grey mouflon, grey, light katmoget, katmoget or gulmoget, other solid colour
Set 2. Black or brown
Set 3. Plain or spotted
Each sheep has two genes of each set. So I've put what we know and ? if we don't know what the second one is.
So your Grey katmoget has 1. Katmoget/? 2. Black/brown 3. Plain/?
The reason we know she's 2. Black/brown is that she herself is showing black (grey katmogets are basically black with the katmoget pattern on top) but must have a brown gene which she's passed on to her lambs.
Your moorit katmoget lambs are: 1. Katmoget/? 2. brown/brown 3. Plain/?
Going away to think about that brownish lamb with black legs!